Warm-bodied fish such as tunas and some sharks may be among the ocean’s most powerful predators, but are likely to face increasing risks in climate breakdown.
Research led by scientists at Trinity College Dublin (TCD) in collaboration with Dr Edward (Ned) Snelling, an experimental physiologist in the University of Pretoria’s (UP) Faculty of Veterinary Science, says these species will be driven towards the poles due to their high fuel demands, physiology and declining food resources.
The study, published today in leading international journal Science, explains that warm-bodied fish such as tunas and some sharks, including the legendary Great White and Ireland’s protected basking shark, burn nearly four times more energy than their cold-blooded counterparts.
This means they are likely to face an increasing risk of overheating as oceans warm, which may result in a reduction of suitable habitat and an enforced relocation towards the poles, the research states.
The study focuses on “mesothermic” fishes, a rare group comprising fewer than 0.1% of all fish species, which can retain metabolic heat and keep parts of their bodies warmer than the surrounding seawater.
This ability has evolved independently several times in some sharks and tunas, enabling higher swimming speeds, long-distance migrations, and enhanced predatory performance, the scientists explain.
The TCD and UP scientists say they developed a novel way to estimate metabolic rate in free-swimming fish.
“By analysing biologging data—from tiny sensors that record body and water temperatures—the team calculated how much heat fish produce and lose in real time. They combined these new measurements, including data from huge basking sharks weighing up to 3.5 tonnes, with hundreds of lab measurements from smaller species,” TCD states.
Dr Nicholas Payne of TCD’s School of Natural Sciences, who is first author of the research paper, said the results” were really quite striking”.
“After accounting for body size and temperature, we found that mesothermic fishes use about 3.8 times more energy than similarly sized ‘ectothermic’, or ‘cold-blooded’ fishes. In addition, a 10°C increase in body temperature more than doubles a fish’s routine metabolic rate which, in practical terms, means warm-bodied predators must consume far more food to fuel their lifestyle,” he said.
“But that heighted energy demand is only part of the story because as fish grow larger their bodies generate heat faster than they can lose it,” Dr Payne said.
“This creates a mismatch driven by basic geometry and physics because bigger bodies retain heat more effectively, and in mesotherms, high metabolic rates amplify this effect,” he explained.
The team found that larger fish become increasingly “warm-bodied” simply because of this imbalance, and it is this scaling mismatch that creates an overheating dilemma with significant implications for these species.
Prof Andrew Jackson, also of TCD’s School of Natural Sciences and senior author of the research paper, said that they were “able to create theoretical ‘heat-balance thresholds’, which are the water temperatures above which large fish cannot shed heat quickly enough to maintain stable body temperatures without changing their behaviour or physiology”.
“For example, a 1-tonne warm-bodied shark may struggle to remain in heat balance in waters above about 17°C,”he said.
“Above such thresholds, fish must slow down, alter blood flow, or dive into cooler depths to avoid dangerous warming but that comes at a cost too; it might be harder to find food, or catch it, for example – especially if your main weapon is speed and power,” Prof Jackson said.
These findings seemingly help to explain long-observed patterns in the ocean, where large fishes tend to occur in cooler waters, at higher latitudes, or at greater depths. They also migrate seasonally, tracking favourable temperatures.
The scientists predict that under future warming scenarios suitable habitat for large mesotherms will shrink, and particularly so during summer months.
They say that while some species, such as Atlantic bluefin tuna, can temporarily increase heat loss or dive to cooler waters, even they may be pushed to their limits if surface waters continue to warm.
Dr Snelling of the University of Pretoria says the research shows that “being a high-performance predator in the ocean comes at a greater cost than we previously appreciated”.
“ As the oceans warm, these species are being pushed closer to their physiological limits, which could have consequences for where they can live and how they survive.”
“What’s particularly concerning is that these animals are already operating on a tight energy budget, and climate change is narrowing their options even further. Understanding these constraints is essential if we want to predict how marine ecosystems will shift in the coming decades,”Dr Snelling added.
“The implications are really sobering as this new finding essentially places these animals in ‘double jeopardy,” Dr Payne observed.
“Many mesothermic fishes are already heavily impacted by overfishing of themselves and also their prey species, so their elevated energy needs make them especially vulnerable when their food becomes scarce.”
“Fossil evidence suggests that warm-bodied marine giants, like the infamous extinct Megalodon shark, suffered disproportionately during past climate shifts when seas changed and today’s oceans are changing at unprecedented speeds, so the alarm bells are ringing loudly at this point,” Dr Payne noted.
This research was supported by funding from Research Ireland, the Marine Institute, Future Legend Films, Oregon State University, and a National Geographic and Human Frontiers Science Project.
